1. Field of the Disclosure
The disclosure relates to a method for quantification of analytes in a sample comprising titanium, tin and/or silicon tetrachloride using inductively coupled plasma mass spectrometry (ICP-MS), particularly the quantification of inorganic analytes such as lead, arsenic and antimony. This disclosure also relates to a method for forming a stable aqueous solution of a sample comprising titanium, tin and/or silicon tetrachloride.
2. Description of the Related Art
Arsenic and antimony can be present in ores from which a titanium, tin or silicon tetrachloride composition useful as a starting material is derived. Most ores contain a variety of impurities that can end up in the compounds produced from them, such as the production of titanium tetrachloride from carbochlorination. Metals can also be useful in producing chlorides such as in the chlorination of tin to form tin tetrachloride. Tin is produced from tin-containing ore such as cassiterite which can contain arsenic and antimony.
It is useful to purify the titanium, tin or silicon tetrachloride composition to remove these and other impurities so that they do not appear in the products made from the titanium, tin or silicon tetrachloride. In particular, it can be especially useful to remove impurities from the titanium tetrachloride that may be used for making oxides, especially certain types of pigments (such as pearlescent titanium dioxide pigments); titanium metal; and catalysts. It is also useful to remove the impurities from tin tetrachloride that may be used in oxides, catalysts, or the production of organotin compounds. Silicon tetrachloride purity is useful in many applications such as in oxides such as fumed silica, ceramics or combined forms such as with titanium dioxide pigment.
In order to know the level of impurities in a sample or whether impurities have been effectively reduced, including to a trace amount or an amount on the order of parts per billion, special analytical techniques have been developed. One such analytical technique is inductively coupled plasma mass spectrometry (ICP-MS).
However, certain compositions such as titanium, tin and silicon tetrachloride pose challenges in the ICP-MS analysis. Neat titanium tetrachloride for example made by the chlorination of titanium-bearing ores, such as ilmenite or rutile, can be challenging to analyze using ICP-MS because it is known to be corrosive and water-sensitive. Tin and silicon tetrachloride are also known to be corrosive and water-sensitive.
One known method for analyzing for arsenic and antimony in titanium tetrachloride at trace levels is the hydride generation method using typically Graphite Furnace Atomic Absorption Spectroscopy (GFAA) but also Atomic Absorption Spectroscopy (AA) or Inductively Coupled Plasma—Atomic Emission Spectroscopy (ICP-AES). Inductively coupled plasma mass spectrometry (ICP-MS) can also be used. Usually for the hydride generation method the titanium tetrachloride is pretreated to avoid the corrosion and water sensitivity problems, and chloride interferences, by first converting it to titanium dioxide then digesting the titanium dioxide.
In the hydride generation method, elements of interest (As, Sb, Pb, Sn) form volatile hydrides at ambient temperatures under reducing conditions. The acid-borohydride reaction that is used most frequently for the hydride generation method is:
where E is the hydride-forming element and m may or may not equal n. In contrast to conventional solution nebulization, the hydride-forming elements enter the instrument (in the gaseous phase using a carrier gas such as N2 or Ar). This serves to: 1) preconcentrate the analyte, 2) produce ions more efficiently, and 3) minimize potential spectral interferences due to matrix separation. This method is known for obtaining acceptable detection limits for solutions such as drinking water, soil extractions, or digestions of ore or other inorganic materials.
Several problems exist with the hydride-generation method, however. The hydride-generation method does not work for all elements of interest, especially including vanadium, iron, copper, niobium, and mercury, among others, thereby requiring a second instrumental method for their determination. Also, undesirable hydride gases such as AsH3 can be produced in the hydride-generation method. The proper preparation steps and handling of the analyte gases must also be very rigorously followed to avoid losses that would underestimate results. Moreover, the additional pretreatment steps for converting TiCl4 to titanium dioxide then digesting the titanium dioxide significantly increase the time required to complete the analysis and add additional opportunities for sample preparation error and contamination.
ICP-MS is a well known analytical technique for detecting elements in a liquid sample. In the ICP-MS technique, typically the liquid sample is first nebulized, then ionized in a plasma and the resulting ion beam is extracted under vacuum into a collision or reaction cell that is positioned before an analyzer quadrupole. A collision gas (such as helium) or a reaction gas (such as hydrogen) is then introduced into the cell which consists of a multipole (such as a quadrupole, hexapole or octapole), usually operated in the radio frequency (rf)-only mode. The rf-only field does not separate the masses like a traditional quadrupole, but instead has the effect of focusing the ions while they contact the gas for reactions or collisions. When in reaction mode, by a number of different reaction mechanisms, interfering ions are converted to either noninterfering species or another ion which has a different nominal mass than the ions of interest. When in collision mode, larger polyatomic ions will undergo more collisions than monoatomic ions of the same nominal mass, causing them to lose more energy. An energy filter at the cell exit prevents the polyatomic ions from reaching the detector and causing interferences. ICP-MS and the reaction-collision cell are described in U.S. Pat. Nos. 6,140,638 of Tanner et al. and 6,875,618 of Bandura et al.
There is a need for a method for analyzing impurities, especially in titanium tetrachloride compositions, that avoids the need for hydride generation and formation of undesirable hydride gases, a second instrumental method for analyzing impurities such as vanadium, iron, copper, niobium, and mercury that cannot be detected by the known hydride-generation method, losses from preparing analyte gases outside the analytical instrument, and high concentrations of interference producing ions.
Using ICP-MS for detecting impurities in titanium tetrachloride poses a problem because the liquid titanium tetrachloride must be formed into a solution that is safe to handle and compatible with the elemental analysis instrumentation. Tin and silicon tetrachloride pose similar challenges. U.S. Pat. No. 6,444,189 of Wang et al. discloses a method for preparing an aqueous solution by reacting the pure titanium tetrachloride with water. This technique cannot prevent the loss of the more volatile impurities in making up the solution. Also, with this method it is difficult to have tight control over the specific gravity of the final solution making reproducibility difficult. It is also more difficult to know the exact masses of the analytes of interest in the starting solution compared to the final composition. For ICP-MS analysis, sample analysis time will also be greatly increased if the samples are not similar in matrix, viscosity, and specific gravity, since matrix matching between calibration standards and samples is required when analyzing multiple solutions together.
A solution suitable for ICP analysis could also be obtained by reacting TiCl4 with a suitable alcohol to form a titanate solution. However, the alcohol must be chosen appropriately for safety. This method has the same problems as the addition directly to water described in Wang et al., where control of the resulting concentration is difficult. The preparation method will also directly impact the types of metals that can be accurately quantified because of the different reaction chemistries. The titanate solution can be sensitive to precipitation when reacted with water which can make the preparation of the baseline standards problematic. U.S. Pat. No. 5,350,644 discloses making a stabilized titanium solution from a titanate solution. The stabilization adds yet another layer of complication in the sample preparation, plus another set of potential interferents.
Titanium tetrachloride can also be converted into an aqueous titanyl sulfate solution through precipitation with sulfuric acid and being re-dissolved in water. This procedure is not quantitative for all potential elements of interest. It also leaves residual chlorides present, has difficulty in reproducibility, can result in the formation of insoluble sulfates, and can impact sensitivity. The same is also true for conversion to a stabilized oxalic acid solution such as is discussed in U.S. Pat. No. 5,776,239. All these techniques add steps for analysis and pose problems associated with converting the titanium tetrachloride composition.
Methods that remove chloride interferences by converting the titanium tetrachloride into a solid compound such as titanium dioxide or titanyl sulfate add additional steps which can cause quantitative errors, increase the potential for measurement errors and add to the processing time.